This invention relates in general to differential gear mechanisms and in particular to an improved structure for a planetary gear differential which is selectively operable in either an engaged mode, wherein a driving connection is provided therethrough, and in a disengaged mode, wherein no driving connection is provided.
Differential gear mechanisms, commonly referred to simply as differentials, are well known devices which are frequently used in the drive train systems of most vehicles. The differential is usually connected between an input driving shaft (typically a drive shaft from the vehicle engine) and a pair of output driven shafts (typically a pair of axle shafts connected to the vehicle wheels). The differential distributes torque from the input shaft equally to the two output shafts, while permitting such output shafts to rotate at different speeds under certain conditions. As a result, torque is supplied to both wheels of the vehicle as it negotiates a turn, while permitting the outside wheel to turn faster than the inside wheel.
Differentials are commonly used in four wheel drive systems, wherein a transfer case divides power from the vehicle engine between a primary pair of wheels and a secondary pair of wheels. In these systems, a differential is provided for each of the primary and secondary pairs of wheels. During four wheel drive operation, power is transmitted from the transfer case through the primary and secondary differentials to the primary and secondary pairs of wheels, respectively, in the manner described above. Four wheel drive systems of this general type are commonly found in off road vehicles because all four of the vehicle wheels are powered to drive the vehicle on low traction surfaces.
Some four wheel drive systems provide a mechanism for disconnecting the driving connection between the transfer case and the secondary differential such that the secondary pair of wheels is not driven by the vehicle engine. These part-time four wheel drive systems are advantageous because four wheel drive operation is undesirable when the vehicle is driven on roads or other high traction surfaces. In those situations, four wheel drive operation is inefficient from a fuel economy standpoint and causes unnecessary wear. By providing a transfer case with a mechanism for disconnecting the driving connection between the transfer case and the secondary differential, the vehicle can be operated efficiently in both on-road and off-road situations.
Unfortunately, even when the driving connection between the transfer case and the secondary differential is disconnected, the secondary pair of wheels back-drives the components of the secondary differential. This back driving action is undesirable for the same reasons described above. Thus, it is known to provide an internal mechanism within the secondary differential for disconnecting the driving connection between some or all of the components therein and the secondary pair of wheels. As a result, the secondary pair of wheels is permitted to rotate freely and create little drag on the vehicle engine.